1. Field of the Invention
The present invention relates generally to control rod arrangements for nuclear reactors, particularly nuclear reactors of the gas-cooled type having a core pile made up of spherical fissionable fuel elements encased in a reflector chamber. More particularly, it relates to a new distribution arrangement of the control rods in the core pile of such a reactor, whereby a more even distribution flow of neutrons across the diameter of the reactor is obtained, and an overly severe concentration in the flow of neutrons on the periphery of the core is avoided.
2. Description of the Prior Art
There have been several different approaches taken to the design of nuclear reactors. The present invention relates generally to nuclear reactors of the gas-cooled type, wherein the core pile is encased in a reflector chamber consisting of a ceiling dome, a cylindrical wall, and a floor, and into which chamber control rods for the core pile may be introduced for the control and fine adjustment of the reactor. In the present invention the core pile is made up of spherical fissionable fuel elements, the structure thus far described being known to the art.
It is customary to use absorbent graphite rods for the control and fine adjustment of such a nuclear reactor. Depending upon the depth of penetration of the control rods into the core pile, the absorbent graphite will either increase or diminish the the flow of neutrons, and consequently the power output of the core. It is also a known fact that in single-core construction the radial flow of neutrons follows a Bessel function, while the axial flow follows a cosine function.
In general, the object in such a nuclear reactor is to attain a constant and even flow of power output, which depends upon the profile of the neutron flow and the locally varying density of fissionable material. In commercial reactors, various methods are applied to attain an even power output in the radial direction and at the same time attain a maximum output from any given volume of the core pile, while maintaining a certain maximum temperature of the core elements. One of the present methods applied for equalizing the output lies in the designing of the reactor core in such a manner that it is divided into an inner and an outer core, the inner core being of a lesser density of fissionable fuel than the outer.
The absorption or control rods are also utilized to attain an even spherical distribution of output. In commercial reactors the absorption rods are generally distributed evenly over the core in a predetermined pattern, permitting the introduction of rods as may be required, in a variety of central or external positions. Furthermore, it is favorable if the rods can be moved to a position close to the bottom reflector of the reactor structure based upon the premise that an efficient distribution of the neutron flow can be attained only if the control rods are completely immersed.
In the invention described in German Auslegeschrift No. 1,049,986, it was further determined that evenness and and uniformity in the neutron flow can be attained with particular efficiency if the length of the absorbing damper or control rods is approximately proportional to the thermal flow of neutrons. An especially even distribution of neutron flow results from following this approach.
The statements thus far made regarding the general performance of the reactor apply under normal, conventional methods of control and fine adjustment. Computer-derived projections, however, show that when absorption or control rods are introduced at their full length in the core of a gas-cooled reactor moderated by graphite, an excessive concentration of thermal flow of neutrons will occur on the periphery of the core, as well as on the lateral reflector of the reactor structure. The radial distribution of neutron flow is thereby proven to be unfavorable.
There is thus a need for controlling neutron flow at the periphery of a reactor core, and to assure an even distribution of neutron flow across the core. The control rod arrangement of the present invention is intended to satisfy that need.